The present invention generally concerns an oscillator circuit with an inverter amplifier (herein after called an inverter oscillator circuit) having reduced consumption. More particularly, the present invention concerns a relatively low frequency inverter oscillator circuit preferably including a quartz resonator arranged to vibrate according to a torsional vibration mode.
There is already known from European Patent No. EP 1 111 770 A1, in the name of Eta SA Fabriques d'Ebauches and EM Microelectronic-Marin SA, a low frequency quartz oscillator device having improved thermal behaviour. This Application, which is incorporated herein by reference, discloses an inverter type oscillator including a specific quartz resonator arranged to vibrate according to a torsional vibration mode. This specific resonator, which is disclosed in the article by Messrs. Roger Bourquin and Philippe Truchot, “Barreau de quartz vibrant en mode de torsion, Application aux capteurs”, 6th European Chronometry Congress, Bienne, 17–18 Oct. 1996 (cited and incorporated by reference in the aforementioned European Patent Application), has a single cut angle defined by a rotation about the crystallographic axis X of the quartz crystal, and includes, in particular, an undesired fundamental flexural vibration mode located at a first frequency and a desired torsional vibration mode located at a second frequency higher than the first frequency of the undesired flexural vibration mode.
The circuit for maintaining the resonator's oscillation is of the inverter type. In order to ensure that the resonator vibrates according to the desired fundamental torsional vibration mode, and not according to the undesired fundamental flexural vibration mode, the inverter circuit is designed to have a transconductance value such that the limit conditions for ensuring vibration (or minimum and maximum critical transconductance values) are satisfied for the fundamental torsional vibration mode and not for the fundamental flexural vibration mode.
The thermal behaviour of the oscillator device thus designed is therefore greatly improved with respect to conventional oscillator devices and has, in particular, similar thermal behaviour to circuits using AT cut resonators for a considerably lower operating frequency (typically 393 kHz for the torsional resonator compared to 4 MHz for the AT cut resonator) and thus a comparatively lower consumption.